The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2 and the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous organic electrolyte systems.
The term "nonaqueous organic electrolyte" in the prior art refers to an electrolyte which is composed of a solute, for example, a salt or a complex salt of Group 1-A, Group II-A or Group III-A elements of the Periodic Table, dissolved in an appropriate nonaqueous organic solvent. Conventional solvents include propylene carbonate, 1,2-dimethoxyethane (DME), tetrahydrofuran, ethylene carbonate, 3-methyl-2-oxazolidone (3Me2Ox), 3,5-dimethylisoxazole (DMI) or the like.
A multitude of solutes is known and recommended for use but the selection of a suitable solvent has been particularly troublesome, since many of those solvents which are used to prepare electrolytes sufficiently conductive to permit effective ion migration through the solution are reactive with the highly reactive anodes described above. Most investigators in this area, in search of suitable solvents, have concentrated on aliphatic and aromatic nitrogen- and oxygen-containing compounds. The results of this search have not been entirely satisfactory since many of the solvents investigated still could not be used effectively with extremely high energy density cathode materials and were sufficiently corrosive to lithium anodes to prevent efficient performance over any length of time. However, many solvents have been found that are suitable for use with high energy density cathode materials.
U.S. Pat. No. 4,753,859 discloses a nonaqueous lithium cell that utilizes an electrolyte that contains such substances as ethylene carbonate, propylene carbonate and one or more polyethylene glycol dialkyl ethers. U.S. Pat. No. 3,796,604 and U.S. Pat. No. 3,796,605 discloses a lithium base electrochemical generator that utilizes a solvent for the electrolyte that comprises a mixture of tetrahydrofuran and dimethoxyethane or a mixture of tetrahydrofuran and diethylene glycol dimethyl ether.
U.S. Pat. No. 3,915,743 discloses a cell having a negative lithium electrode and an organic electrolyte comprising boron trifluoride and a mixture of an alkyl carbonate and an ether such as dimethyl carbonate and 1,2-dimethoxyethane.
U S. Pat. No. 3,546,022 discloses a cell employing a sodium anode and a liquid electrolyte comprising a solution of alkali metal hexafluorophosphate in a dialkyl ether of an alkylene glycol where the alkyl contains 1 to 4 carbon atoms and the alkylene is selected from the group consisting of diethylene, ethylene, dimethylene and trimethylene.
U.S. Pat. No. 3,928,067 discloses a lithium battery that utilizes an electrolyte comprising a solvent such as propylene carbonate, butyrolactone, ethylene carbonate, dimethylsulfite, acetonitrile and dimethylsulfoxide along with a dopant such as a polyalkylene glycol ether.
U.S Pat. No. 3,996,069 discloses a nonaqueous cell utilizing a highly active metal anode, such as lithium, a solid cathode selected from the group consisting of FeS.sub.2, Co.sub.3 O.sub.4, V.sub.2 O.sub.5, Pb.sub.3 O.sub.4, In.sub.2 S.sub.3 and CoS.sub.2, and a liquid organic electrolyte consisting essentially of 3-methyl-2-oxazolidone in combination with a low viscosity cosolvent, such as dioxolane, and a metal salt selected, for example, from the group consisting of MSCN, MCF.sub.3 SO.sub.3, MBF.sub.4, MClO.sub.4 and MM'F.sub.6 wherein M is lithium, sodium or potassium and M' is phosphorus, arsenic or antimony.
While the theoretical energy, i.e., the electrical energy potentially available from a selected anode-cathode couple, is relatively easy to calculate, there is a need to choose a nonaqueous electrolyte for such couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a selected couple. Thus a cell must be considered as a unit having three parts--a cathode, an anode and an electrolyte --and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell to produce an efficient and workable cell.
In an article titled "The Behaviour Of Lithium Batteries In A Fire" by A. Attewell (Journal of Power Sources, 26 (1989) Pg. 195-200) it concludes that "Under warehouse conditions, the major contributor to a lithium battery fire is, when Present, the flammable electrolyte. Lithium metal itself makes only a minor contribution." Consequently, in many applications a low flammability electrolyte would be desirable for cells using a lithium anode. The electrolyte generally chosen for a lithium cell often represents a compromise in that the advantage of a low flammability electrolyte generally is offset by Poorer performance of the cell in some other area, such as high rate performance, particularly at a low temperatures. In devices such as flash cameras, high rate capability is required and therefore if a cell has poor high rate performance, the cell will not be suitable for use in this type of application.
It is an object of the present invention to provide a low flammability electrolyte solution for use in an electrochemical cell that can be stored at high temperatures for extended periods of time without the cell impedance increasing to levels which substantially reduce cell performance.
Another object of the present invention is to provide a low flammability electrolyte solution for an electrochemical cell employing a mixture of 3-methyl-2-oxazolidone (3Me2Ox) and at least one polyalkylene glycol ether having the formula: EQU R--O(--CH.sub.2 --CH.sub.2 --O).sub.n --R.sup.1
where R and R.sup.1 are C.sub.1 -C.sub.4 alkyl groups and n is greater than one.
Another object of the present invention is to provide a low flammability electrolyte solution that is ideally suited for cells employing a lithium anode and an iron sulfide-containing cathode.
The foregoing and additional object will become more fully apparent from the following description.